1. Field of the Invention
The present invention relates to a light-emitting display, and more particularly, to an organic electroluminescence display configured to prevent line mura generated due to differences in driving transistor characteristics resulting from non-uniformities due to excimer laser annealing.
2. Description of Related Technology
Various flat panel displays have been developed which overcome the large weight and volume which are drawbacks of cathode ray tube displays. Types of flat panel displays include a liquid crystal display, a field emission display, a plasma display panel, and an organic electroluminescence display.
The organic electroluminescence display uses a self-light emitting device for emitting light by recombination of electrons and holes. The organic electroluminescence display has a fast response speed and low power consumption.
FIG. 1 is a schematic diagram of a plurality of pixels in an exemplary organic electroluminescence display, and FIG. 2 is an illustration of the layout of the organic electroluminescence display of FIG. 1.
Referring to FIG. 1 and FIG. 2, the exemplary organic electroluminescence display comprises a plurality of pixel circuits 11 surrounded by a plurality of scanning lines S1 to Sm, a plurality of data lines D1 to Dn, and a first power supply line VDD. The scanning lines Sm are formed in a row direction, and the data lines Dn and the first power supply line VDD are formed in a column direction.
Each pixel circuit 11 comprises an organic light-emitting diode (OLED), a driving transistor MD, a capacitor Cst and a switching transistor MS, wherein the transistors MD and MS are MOSFETs (metal-oxide semiconductor field effect transistors). Each pixel circuit 11 receives a data signal from the data line Dn when a selection signal is supplied to the scanning line Sm, and the pixel emits light in response to the received data signal.
A first electrode of the OLED, e.g., an anode electrode, is connected to the driving transistor, and a second electrode, e.g., a cathode electrode is connected to a second power supply line VSS. The OLED comprises an emitting layer, an electron transport layer, and a hole transport layer, formed between the anode electrode and the cathode electrode. The OLED further comprises an electron injection layer and a hole injection layer. When a voltage is applied between the anode electrode and the cathode electrode of the OLED, electrons generated from the cathode electrode move to the emitting layer via the electron injection layer and the electron transport layer, and electrons generated from the anode electrode move to the emitting layer via the hole injection layer and the hole transport layer. Thereby, electrons from the electron transport layer and holes from the hole transport layer are recombined in the emitting layer to emit light.
In operation, the switching transistor MS is turned on in response to a selection signal supplied to the scanning line Sm, and the switching transistor MS supplies a data signal from the data line Dn to a gate electrode of the driving transistor MD. The storage capacitor Cst stores a voltage difference between a driving voltage supplied to the first power supply line VDD and a data signal supplied to the gate electrode of the driving transistor MS.
The driving transistor MD controls the light emission from the OLED in response to the data signal supplied to the gate electrode of the driving transistor MD, thereby controlling a current level supplied to the OLED from the first power supply line VDD. When the switching transistor MS is turned off, the driving transistor MD supplies a constant current to the OLED with voltage stored in the storage capacitor Cst until a data signal of a next video frame is supplied to the data line Dn. Thereby, the driving transistor controls emission of the OLED.
As described above, the driving transistor MD of the respective pixel circuits 11 plays an important role in controlling the light emission from the OLED. The light emission of the OLED is controlled by the driving transistor MD by controlling the amount of current supplied to the OLED according to voltage supplied to the gate electrode of the driving transistor MD itself. Thus, a current Ids supplied to the OLED through the driving transistor MD is determined by the following equation (1), where W and L are the width and length of channels in the driving transistor MD, Vgs is the voltage applied across the gate and source terminals of the driving transistor MD, Vth is the threshold voltage of the driving transistor, μ is mobility, and Cox is gate capacity per unit area of the driving transistor MD:Ids=(1/2)×(W/L)×μCox(Vgs−Vth)2  (1)
Referring to equation (1), the current Ids supplied through the driving transistor MD is determined by data voltage supplied to the gate electrode of the driving transistor MD, and characteristics of the data voltage depend on the threshold voltage Vth and mobility. The driving transistor MD, however, may have non-uniform characteristics, such as threshold voltage and mobility, as a result of a laser annealing process comprising crystallizing amorphous silicon into polycrystalline silicon. In a process for fabricating an organic electroluminescence display, a process of forming a semiconductor layer for the pixel circuit transistors MD and MS includes a laser annealing process comprising crystallizing amorphous-silicon thin film into a poly-silicon thin film.
FIG. 3 is an illustration of a laser annealing method for crystallizing a semiconductor layer for the transistors of FIG. 2, and FIG. 4 is an illustration of a line mura generated in a conventional organic electroluminescence display. The line mura denotes non-uniformity of displayed image. Referring to FIG. 3, a poly-silicon thin film is formed by crystallizing an amorphous-silicon thin film patterned on a substrate 10. The amorphous-silicon thin film is crystallized by an excimer laser annealing process (labeled “ELA”), comprising scanning a line beam 40 using an excimer laser in a row direction. The poly-silicon thin film is formed by repeatedly melting and solidifying the amorphous-silicon thin film by a very short laser beam that is irradiated at high energy, thereby recrystallizing the amorphous-silicon thin film.
Although the laser annealing process provides for formation of a poly-silicon thin film on a wide substrate, grain size and mobility are varied according to deviation of beam energy density generated per laser irradiation time points. Accordingly, characteristics of the poly-silicon thin film become non-uniform along a column direction perpendicular to a laser scan direction. Thus, when the poly-silicon thin film is used as the semiconductor layer of the driving transistor MD, transistor characteristics such as threshold voltage and mobility are non-uniform in a column direction. These transistor non-uniformities may result in luminance deviation in a column direction relative to the same luminance. As a result, a line mura 42 is generated perpendicular to a laser scan direction, as illustrated in FIG. 4, due to non-uniform characteristics of the driving transistors MD of the display pixels. The line mura 42 reduces image quality due to high visibility, and reduces yield of an organic electroluminescence display.